1. Field of the Invention
This invention relates generally to the diagnosis of cancer and specifically to identification of a hypermethylated glutathione-S-transferase (GSTP1) promoter as a diagnostic indicator of prostatic tissue cellular proliferative disorder.
2. Description of Related Art
Current cancer tests are often nonspecific and therefore of limited clinical utility. For example, a biochemical test, widely used for both the diagnosis and monitoring of cancer, measures levels of carcinoembryonic antigen (CEA). CEA is an oncofetal antigen detectable in large amounts in embryonal tissue, but in small amounts in normal adult tissues. Serum of patients with certain gastrointestinal cancers contain elevated CEA levels that can be measured by immunological methods. The amount of CEA in serum correlates with the remission or relapse of these tumors, with the levels decreasing abruptly after surgical removal of the tumor. The return of elevated CEA levels signifies a return of malignant cells. CEA, however, is also a normal glycoprotein found at low levels in nearly all adults. Moreover, the relevance of CEA is severely compromised by the fact that this protein can be elevated in nonmalignant conditions and not elevated in many cancers. Therefore CEA is far from ideal as a cancer marker.
Human cancer cells typically contain somatically altered genomes, characterized by mutation, amplification, or deletion of critical genes. In addition, the DNA template from human cancer cells often displays somatic changes in DNA methylation (E. R. Fearon, et al., Cell, 61:759, 1990; P. A. Jones, et al., Cancer Res., 46:461, 1986; R. Holliday, Science, 238:163, 1987; A. De Bustros, et al., Proc. Natl. Acad. Sci., USA, 85:5693, 1988); P. A. Jones, et al., Adv. Cancer Res., 54:1, 1990; S. B. Baylin, et al., Cancer Cells, 3:383, 1991; M. Makos, et al., Proc. Natl. Acad. Sci., USA, 89:1929, 1992; N. Ohtani-Fujita, et al., Oncogene, 8:1063, 1993). However, the precise role of abnormal DNA methylation in human tumorigenesis has not been established. DNA methylases transfer methyl groups from the universal methyl donor S-adenosyl methionine to specific sites on the DNA. Several biological functions have been attributed to the methylated bases in DNA. The most established biological function is the protection of the DNA from digestion by cognate restriction enzymes. The restriction modification phenomenon has, so far, been observed only in bacteria. Mammalian cells, however, possess a different methylase that exclusively methylates cytosine residues on the DNA, that are 5' neighbors of guanine (CpG). This methylation has been shown by several lines of evidence to play a role in gene activity, cell differentiation, tumorigenesis, X-chromosome inactivation, genomic imprinting and other major biological processes. (Razin, A., H., and Riggs, R. D. eds. in DNA Methylation Biochemistry and Biological Significance, Springer-Verlag, N.Y., 1984).
Glutathione S-transferases (GSTs) catalyze intracellular detoxification reactions, including the inactivation of electrophilic carcinogens, by conjugating chemically-reactive electrophiles to glutathione (C. B. Pickett, et al., Annu. Rev. Blocbern., 58:743, 1989; B. Coles, et al., CRC Crit. Rev. Biochem. Mol. Biol., 25:47, 1990; T. H. Rushmore, et al., J. Biol. Chem. 268:11475, 1993). Human GSTs, encoded by several different genes at different loci, have been classified into four families which have been referred to as .alpha., .mu., .pi., and theta (B. Mannervik, et al., Biochem. J., 282:305, 1992).
Studies have shown that many human cancers exhibit increased GSTP1 expression relative to their tissues of origin (S. Tsuchida, et al., CRC Crit. Rev. Biochem. Mol. Biol., 27:337, 1992). The limitation and failings of the prior art to provide meaningful markers which correlate with the presence of cell proliferative disorders, such as cancer, has created a need for markers which can be used to diagnose and monitor the course of such disorders. The present invention fulfills such a need.